1. Field of the Invention
This invention relates to a percussion hammer such as an electric hammer drill which has a motion converting mechanism, a striking mechanism, and a rotation transmitting mechanism.
2. Description of the Prior Art
Japanese published examined patent application 61-19395 (corresponding to U.S. Pat. No. 4,236,588) and Japanese published unexamined patent application 4-500043 (corresponding to U.S. Pat. No. 5,111,890) disclose hammer drills in which a movable coupling member is located between a drive wheel and a drive cylinder which engages a tool holder. The coupling member is changed among three different positions by an adjustment member. When the coupling member assumes a first position, the drive wheel and the drive cylinder are connected via the coupling member so that rotational movement is transmitted from the drive wheel to the drive cylinder. When the coupling member assumes a second position, the drive wheel and the drive cylinder are disconnected from each other so that the transmission of rotational movement therebetween is interrupted. When the coupling member assumes a third position, the drive cylinder is disconnected from the drive wheel and is simultaneously locked to a casing of the hammer drill by the coupling member.
During operation of the above-indicated hammer drills in which the coupling member is changed between the first position and the third position, loads or forces of significantly different levels act on the connection between the coupling member and the drive wheel and the connection between the coupling member and the casing respectively. Accordingly, it is generally difficult to optimally design the coupling member with respect to such loads or forces of different levels.
In the above-indicated hammer drills, the coupling member has a first engagement portion for providing the connection with the drive wheel and a second engagement portion for providing the connection with the casing. If the first engagement portion of the coupling member breaks while the second engagement portion thereof is normal, it is necessary to replace the whole of the coupling member.